1. Field of Invention
The invention generally relates to methods and apparatuses for controlling or otherwise suppressing transients and their applications to various optical communication networks.
2. Description of Related Art
Optical WDM (wavelength division multiplexed) and other types of optical transmission systems are becoming increasingly dynamic. For example, there is a greater emphasis on dynamic add/drop architectures, protection switching, and traffic rerouting all of which can introduce unwanted optical power transients. In such systems, optical power transients may also be introduced by various other factors such as routine system activities and unintended events such as fiber cuts and equipment failures. Once an optical power transient is created it is often exacerbated by optical amplifiers and other active optical devices.
The performance of the optical transmission system can be adversely affected particularly by fast power transients, beyond the effects of the temporary change in the power level at the receiver. Both the magnitude and temporal characteristics of the optical power transients that occur can limit the performance of various elements such as optical receivers and amplifiers in the system. While component designers continue to improve the transient performance of individual sources, a comprehensive strategy for suppressing the power transients that exist in these systems offers improved system design flexibility and coincident cost savings.
Rapid changes in optical power levels occur routinely in transmission systems. Reconfigurable optical add/drop architectures are designed to accommodate changing levels in the total optical power present in a given fiber or at the input to various optical components but these accommodations are often insufficient.
Other sources of optical power transients include optical protection switching mechanisms that can lead to changes in the optical power level on the order of the speed of opto-mechanical switches, typically 100 to 200 μsec. Fiber cut events can be faster, reaching a few 10ths of μs. System upgrades and maintenance often require changes in power levels that give rise to optical power transients.
Even a small optical power change from any of these sources can be exacerbated by constant gain amplifiers such as constant gain erbium-doped fiber amplifiers (EDFAs) particularly if the time scale is similar to the time response of the EDFA control loop. The effects would be even worse for optical amplifiers operated in constant power mode. The ultimate power transient after a cascade of EDFAs can be particularly problematic. FIG. 15 shows one example of a conventional optical network configuration where one WDM channel is added at Node 1, and a second WDM channel is added at Node 2. If the fiber between Nodes 1 and 2 is cut, EDFA #3 will experience a rapid 3 dB change (in situations where both signals have the same optical power) in its input power level, and the actual gain of EDFA #3 may experience a brief excursion from its target value due to the finite response time of its control loop.
Similarly, equipment failure or active optical components can introduce sudden changes in the input power at an optical amplifier. An example of the effect of a simulated fiber cut (optical transition time 200 μsec) at the beginning of a cascade of 10 EDFAs is shown in FIG. 16. In this case there are two optical channels present at the input of the cascade (EDFA #2 in FIG. 1), and one of these is switched off to simulate a fiber cut. The change in input power at the first EDFA is 3 dB, and if the EDFA control loop is capable of maintaining constant gain the second channel would be unaffected. However, the small transient (0.5 dB) observed at the output of the first EDFA induces a much larger transient (5 dB) at the output of the tenth amplifier. The performance of the EDFAs is limited by both the fundamental Erbium dynamics and the design and implementation of the EDFA control loop, which has a finite response time. In addition to the temporary power transient, the difference between the initial and final steady-state power levels increases throughout the cascade, due to the input power dependence of the gain accuracy of each EDFA.
The above are non-limiting examples of the situations in which optical power transients arise and how they may be exacerbated. It is to be understood that many other examples and situations exist.